[AMDGPU] Increased vector length for global/constant loads.
Summary: GCN ISA supports instructions that can read 16 consecutive dwords from memory through the scalar data cache; loadstoreVectorizer should take advantage of the wider vector length and pack 16/8 elements of dwords/quadwords.
Author: FarhanaAleen
Reviewed By: rampitec
Subscribers: llvm-commits, AMDGPU
Differential Revision: https://reviews.llvm.org/D43275
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325518
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